1. Field of the Invention
The present invention relates to a camera provided with a device that detects the operating state of the shutter.
2. Description of the Related Art
In the known art, in a single lens reflex camera for general use, the light flux from the subject is stopped with the diaphragm of the lens and the exposure time of the film is restricted with a mechanical shutter. The shutter which is used in this type of single lens reflex camera comprises basically two shutter curtains, the so-called front and back curtains. Before exposure, the front curtain covers the opening of the aperture. When the shutter button is pressed, the front curtain first withdraws from the aperture opening to start the exposure of the film surface and when a specific length of time has elapsed, the opening of the aperture is started to operate so that the aperture opening is covered by the rear curtain. The operation of each curtain is mechanically driven by applied spring force. The operation is initiated by cutting off the power supply to an electromagnet that holds each curtain.
In recent years, high speed shutter capability, such as 1/8000 sec. has become an important specification, and at the same time, higher speeds for the synchronized strobe to enable image-making in a bright environment has become a requirement. For these purposes, it is necessary to increase the force of the aforementioned springs, in order to significantly increase the operating speed of the curtains (hereafter referred to as curtain speed), while it is also necessary to implement control to ensure that the width of the slit formed by the front curtain and the back curtain is narrow.
However, with this type of shutter, in which the curtain speed is high and the slit width is narrow, the following problems arise.
Even if the exposure time is controlled by accurately controlling the timing of the power supply to the electromagnets, it is exactly the same as the prior art in that the actual exposure time is entirely dependent upon the state of each curtain, both of which are mechanically operated. The varying factors affecting shutter time (shutter speed) include the speed at which the connecting-holding mechanism is able to be released by cutting off the power supply to the electromagnet, and inconsistencies in the mechanical operating system, including the springs. Also, as they naturally possess variable characteristics subject to thermal conditions and variations with time, unpredictable overall performance changes occur. This phenomenon becomes even more prominent when higher speed of operation is attempted.
Typical errors include, for example, failure to achieve the desired shutter time, resulting in excessive or insufficient exposure of the film surface. In an extreme case, no exposure at all may be achieved because the operation of the two, curtains overlap. Another problem, the reverse, may arise, in which excessive exposure occurs because the curtains do not close. These problems are exacerbated by the fact that none of these phenomena are detected during photographing and so the failure is not discovered until the film is developed.